Content addressable memory (CAM) devices are frequently used in network switching and routing applications to determine forwarding destinations for data packets. A CAM device can be instructed to compare a selected portion of an incoming packet (e.g., a destination address extracted from the packet header) with CAM words (e.g., forwarding address) stored in an array within the CAM device. If there is a matching entry stored in the CAM array, the index of the matching CAM word can be used to access a corresponding location in an associated memory device to retrieve a destination address and/or other routing information for the packet.
A CAM device includes a CAM array having a plurality of CAM cells organized in a number of rows and columns. Each row of CAM cells, which can be used to store a CAM word, is coupled to a corresponding match line that indicates match results for the row. Each column of CAM cells is typically coupled to one or more data lines or data line pairs that can be used to drive data into a selected CAM row during write operations and/or for providing a search key to the CAM rows during compare operations. During a compare operation, the search key (e.g., the comparand word) is provided to the CAM array and compared with the CAM words stored therein. For each CAM word that matches the search key, a corresponding match line is asserted to indicate the match result. If any of the match lines are asserted, a match flag is asserted to indicate the match condition, and a priority encoder determines the match address or index of the highest priority matching entry in the CAM array. The CAM device outputs a response that identifies whether a match was made and if so, the match address or index of the highest priority matching entry in the CAM array.
CAM arrays typically include either binary CAM cells that store binary data values (i.e., a logic “1” or a logic “0” value) or ternary CAM cells that store ternary data values (i.e., a logic “1” value, a logic “0” values, or a don't care value). Ternary CAM cells store a data bit and a mask bit. For example, when a mask bit within a ternary CAM cell is inactive (e.g., set to a logic 1 value), the ternary CAM cell operates as a conventional binary CAM cell storing an “unmasked” data bit. When the mask bit is active (e.g., set to a logic 0 value), the ternary CAM cell is treated as storing a “don't care” value, which means that all compare operations performed on the actively masked ternary CAM cell will result in a cell match condition.
FIG. 1A shows a well-known quaternary CAM cell 100 of the type described in U.S. Pat. No. 5,319,590 issued to Montoye, the disclosure of which is incorporated herein by reference. Quaternary CAM cells, which are also referred to as XY CAM cells, include two storage cells 120 and 122 coupled to a compare circuit 130. The two data bits X and Y can collectively represent four possible states: “0”, “1”, “don't care”, and a fourth state which may be left unused or may indicate “invalid,” as depicted in FIG. 1B. The logic “0” and “1” states correspond to the logic states represented, for example, by a conventional binary CAM cell. For each of these states, if the comparand data (e.g., provided to CAM cell 100 via complementary comparand lines CL and CLB) matches the data stored in CAM cell 100, compare circuit 130 does not discharge the match line ML, which indicates a match condition. Conversely, if the comparand data does not match the data stored in CAM cell 100, compare circuit 130 discharges ML (e.g., toward ground potential) to indicate the mismatch condition. For the “don't care” state, the logic low values for X and Y maintain respective transistors 132 and 134 in non-conductive states, thereby preventing compare circuit 130 from discharging ML. In this manner, data stored in CAM cell 100 is masked from the compare operation, thereby forcing a match condition for CAM cell 100, regardless of the comparand data. For the “invalid” state, the logic high values for X and Y maintain respective transistors 132 and 134 in conductive states. During a compare operation, one of the complementary comparand data bit pair provided on CL/CLB will be logic high, thereby causing compare circuit 130 to discharge ML to indicate the mismatch state. In this manner, data stored in CAM cell 100 forces a mismatch condition, regardless of the comparand data, and is therefore not normally used.
The format of a data packet may conform to a standard protocol, such as Interlaken. Interlaken is an interconnect protocol which may be used by certain CAM devices. Interlaken is optimized for high-bandwidth and reliable packet transfers. The Interlaken protocol builds on the channelization and per channel flow control features of the SPI-4.2 protocol, while reducing the number of chip I/O pins. In the Interlaken protocol, bundles of serial links create a logical connection between components with multiple channels, backpressure capability, and data-integrity protection to boost the performance of communications equipment.
The Interlaken protocol is silent with respect to how error conditions experienced by a CAM device may be reported. U.S. Pat. No. 6,690,595 (the '595 patent), incorporated herein by reference, is illustrative of a prior art technique for reporting that a CAM device is encountering an error condition. The '595 patent teaches an approach where a CAM device includes a CAM array and an error detection circuit coupled to receive a data value from a selected storage location within the CAM array. The error detection circuit is adapted to generate an error indication according to whether the data value includes an error. An error storage circuit is coupled to receive the error indication from the error detection circuit and is adapted to store an error address that corresponds to the selected storage location if the error indication indicates that the data value includes an error and if the error address is not already stored within the error storage circuit.